Hi,
All backports are deactivated by default (i.e. the packages are pinned to 100 by using ButAutomaticUpgrades: yes in the Release files. If you want to install something from backports run:
apt-get -t jessie-backports install "package"
This seems to be wrong or at least unclear because apt-get install ffmpeg works just fine even though it's in jessie's backports.
There's also a closing bracket missing :P
Would be nice if this could be updated. Thanks.
Max